The present invention relates to a lead frame, particularly a lead frame which uses, as a base material, a base having a metal such as a copper-type metal as the main component and which has bumps made of a metal on the surfaces of inner lead tip portions. The invention also relates to a manufacturing method of such a lead frame, a semiconductor device using such a lead frame, an assembling method of the semiconductor device, and an electronic apparatus using the semiconductor device.
Lead frames are indispensable for the techniques of forming leads of respective electrodes of a semiconductor chip, and commonly used in mounting semiconductor chips. Lead frames are generally made of a copper-type material, and bumps are formed on the surfaces of respective inner lead tip portions so as to be bonded to respective electrode pads on a semiconductor chip. Although previously such bumps were using aluminum, in recent years techniques of forming bumps using gold have been developed. The reasons for forming bumps made of gold are that good bonding performance is obtained and that gang bonding is enabled which provides high production efficiency than single point bonding.
In particular, the fact that gang bonding is enabled is very important for the following reason. Electrode pads of a semiconductor chip are made of aluminum. According to the current technology, single point bonding should be used to bond leads of the previous type having aluminum bumps to such aluminum pads. Where bumps are made of gold, gang bonding can be performed and hence the bonding efficiency can greatly be increased.
FIG. 1 illustrates bonding in a case where bumps are made of gold. Respective inner leads made of copper, for instance, have bumps (having a three-layer structure of gold, tin, and aluminum that are provided in this order from the surface) at the tip portions. The bumps are positioned above respective electrode pads (having a three-layer structure of gold, tin, and aluminum that are provided in this order from the surface) of a semiconductor chip and then bonded thereto by ultrasonic bonding by using a bonding tool.
The conventional techniques of forming bumps made of gold will be described below. In a first technique, the bump-forming side of each inner lead made of copper or the like is selectively half-etched so that a bump-forming portion is protruded. Then, the entire lead surface is plated with gold (a nickel plating layer is formed as an undercoat). In a second technique, gold to constitute a bump is transferred to each inner lead surface by a transfer method. In a third method, gold is evaporated on a bump-forming portion of each inner lead. In a fourth method, the entire surface of each inner lead is plated with a gold film with a nickel plating layer formed as an undercoat.
Each of the above conventional techniques of forming bumps made of gold which provide good bonding performance and enable gang bonding has the following problems. The first technique, in which the bump-forming side of each inner lead is selectively half-etched so that a bump-forming portion is protruded and then the entire lead surface is plated with gold, has a problem that because copper that constitutes leads is hard, electrode pads of an IC chip are prone to cracking. Since a nickel plating layer needs to be formed as an undercoat of a gold plating layer, the number of steps is increased. Further, where gold is formed on the entire lead surface, a large amount of gold is used, to increase the material cost, that is, the overall cost.
The second technique, in which gold to constitute a bump is transferred to each inner lead surface by a transfer method, has a problem that it is prone to positional deviations because of low accuracy of the transfer position control.
The third technique, in which gold is evaporated on a bump-forming portion of each inner lead, has problems that it requires a large investment in equipment because evaporation equipment is expensive, that the productivity is low because gold evaporation takes long time, and that the adhesion between a gold evaporation film and a copper lead is insufficient.
In the fourth technique, in which the entire surface of each inner lead is plated with a gold film with a nickel plating layer formed as an undercoat, has a problem that the number of steps is increased because the nickel plating layer needs to be formed as the undercoat of the gold plating layer. Further, where gold is formed on the entire lead surface, a large amount of gold is used, to increase the material cost. In particular, the gold plating film needs to be formed so as to be thick enough to secure good bonding performance. Forming so thick a gold plating film consumes a very large amount of gold, to cause a non-negligible cost increase.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to improve the positional accuracy of bump forming positions, enable bumps to be formed even when the inner lead pitch is made finer, form in a relatively short time bumps that are thick enough to secure good bonding performance without using evaporation that is low in productivity and requires a high equipment cost, provides sufficient adhesion between bumps and inner leads, improve the bonding performance while preventing an undue increase in material cost by avoiding useless consumption of a metal, enables gang bonding, and reduce the cost of a metal material of bumps in manufacture of a lead frame which uses, for instance, a metal base as a base material and has bumps made of a metal on the surfaces of inner lead tip portions.
The invention provides a first manufacturing method of a lead frame, comprising the steps of forming metal films to constitute bumps by plating on a base made of a metal; and forming, by plating with a metal, a circuit wiring including inner leads so that the inner leads are connected to the respective metal films.
This manufacturing method of a lead frame can increase the adhesion to the underlying member because of the use of plating rather than evaporation, and can shorten the time required for forming metal films of a necessary thickness because of their high growth rate. Further, the investment in equipment can be reduced because the cost of equipment for plating is lower than that for evaporation.
Since metal films to constitute bumps are formed by plating on a highly rigid base that has not been formed with inner leads yet, the underlying member (base member) is more stable than in a case of forming metal films on the tip portions of the surfaces of inner leads already formed.
Therefore, the bumps are easy to form and the accuracy of their forming positions can be increased.
The metal films may be formed by forming, on the base, a resist film having a negative pattern of a pattern of the intended circuit wiring, and performing the plating in a state that tip portions of the respective inner leads are masked by a mask jig so as to be prevented from being plated. In this case, by securing a proper positional relationship between the resist film and the mask jig, the metal films to constitute bumps can easily be formed with a correct positional relationship with inner leads to be formed next so as to be connected to the metal films.
Further, the circuit wiring including the inner leads may be formed thereafter by performing plating by using only the resist film as a mask in a state that the mask jig is removed. In this case, a desired positional relationship between the bumps and the circuit wiring can be established with high accuracy.
The invention also provides a second manufacturing method of a lead frame, comprising the steps of forming metal firms to constitute bumps by plating on a base having, on a surface thereof, an etching stop layer; forming, by plating with a metal, a circuit wiring including inner leads so that the inner leads are connected to the respective metal films; forming, so as to cover the circuit wiring, an insulating film having openings where the circuit wiring is exposed; forming external electrodes in the respective openings by plating; selectively etching a portion, opposite to the circuit wiring, of the base by using the etching stop layer as a stopper for preventing etching of a portion, on a side of the circuit wiring, of the base; and removing the etching stop layer.
In this manufacturing method of a lead frame, metal films to constitute bumps are formed on a base having an etching stop layer on its surface and a circuit wiring is thereafter formed. This makes it possible to apply the technique of the above-described first manufacturing method of a lead frame for forming metal films as bumps to techniques in which inner leads are formed by using, as a base, a structure in which an etching stop layer is formed on the surface of a relatively thick metal layer made of copper or the like that will become the frame portion of a lead frame, that is, an outer ring, or by using a structure in which a material layer for forming a circuit wiring including inner leads is further formed on the surface of the etching stop layer (refer to Japanese Unexamined Patent Publication Nos. Hei. 8-148530 and Hei. 8-47221).
The invention provides a lead frame comprising an insulating film having openings; a wiring film including inner leads formed on one surface of the insulating film; electrodes as external terminals provided on the other surface of the insulating film so as to be connected to the wiring film through the openings of the insulating film; and bumps made of palladium and connected to the respective inner leads. The invention also provides a semiconductor device using this lead frame.
According to the above lead frame and semiconductor device, since the bumps are made of palladium rather than gold, the material cost is low and gang bonding is still possible. In the case of plating with gold, it is necessary to use a plating liquid containing cyan which is very toxic and dangerous. In the case of plating with aluminum, it is necessary to use an explosive plating liquid. (Aluminum is more frequently formed by evaporation than by plating.) In contrast, in the plating with palladium, it is not necessary to use such problematic plating liquids and hence a merit is obtained that the plating is very safe. An example of palladium plating liquids is paragrite SST of Kojundo Chemical Co. Since palladium plating liquids generally contain a toxic component having an ammonia-type pungent smell, they are not completely free of toxicity. However, gold plating liquids for industrial purposes generally contain cyan compounds, which are such toxic substances that even a very small amount thereof is lethal because cyan compounds produce a hydrogen cyanide gas when reacting with hydrochloric acid HCl or the like. Palladium plating liquids are thus much safer than gold plating liquids.
The invention provides an assembling method of a semiconductor device, comprising the steps of manufacturing a lead frame by any of the above manufacturing methods of a lead frame; and bonding, at the bumps, the inner leads of the lead frame to respective electrode pads of a semiconductor chip by single point bonding or gang bonding.
According to this assembling method of a semiconductor device, since a lead frame manufactured by any of the above manufacturing methods of a lead frame is used, a lead frame formed with bumps that has good bonding performance and enables gang bonding can be used in assembling a semiconductor device. Where the inner leads of such a lead frame are bonded to the respective electrode pads of a semiconductor device by single point bonding, bonding can be performed with much superior performance. Where gang bonding is performed, the productivity can be increased.
Further, the invention provides an electronic apparatus comprising a semiconductor device that uses the above-described lead frame.
By virtue of the use of a semiconductor device that is given the above-mentioned advantages of the lead frame, this electronic apparatus benefits from the same advantages.